The use of interferometric methods for three-dimensional inspection of an object or to measure the variations of height (relief) of an object is well known. Generally stated, these methods consist in generating an interferometric pattern on the surface of the object and then analyzing the resulting interferometric image (or interferogram) to obtain the relief of the object. The interferometric image generally includes a series of black and white fringes.
Interferometric methods that require the use of a laser to generate the interferometric pattern are usually called “classic interferometric methods”. In such classic methods, the wavelength of the laser and the configuration of the measuring assembly generally determine the period of the resulting interferogram. Classic interferometry methods are generally used in the visible spectrum to measure height variations in the order of micron.
However, there has been difficulty in using such a method to measure height variations on a surface showing variations in the order of 0.5–1 mm when they are implemented in the visible spectrum. Indeed, the density of the black and white fringes of the resulting interferogram increases, causing the analyzis to be tedious.
Another drawback of classic interferometric methods is that they require measuring assemblies that are particularly sensitive to noise and vibrations.
Three-dimensional inspection methods based on Moiré interferometry allow for a more accurate measurement of the object in the visible spectrum as compared to the accuracy of classic interferometric methods. These methods are based on the analyzis of the frequency beats obtained between 1) a grid positioned over the object to be measured and its shadow on the object (“Shadow Moiré Techniques”) or 2) the projection of a grid on the object, with another grid positioned between the object, and the camera that is used to photograph the resulting interferogram (“Projected Moiré Techniques”). In both cases, the frequency beats between the two grids produce the fringes of the resulting interferogram.
More specifically, the Shadow Moire technique includes the steps of positioning a grid near the object to be measured, providing illumination from a first angle from the plane of the object (for example 45 degrees) and using a camera, positioned at a second angle (for example 90 degrees from the plane of the object), to photograph the interferogram.
Since the distance between the grid and the object varies, this variation of height produces a variation in the pattern of the interferogram. This variation in the pattern can then be analyzed to obtain the relief of the object.
A drawback to the use of a Shadow Moire technique for measuring the relief of an object is that the grid must be very closely positioned to the object in order to yield accurate results, causing restrictions in the set-up of the measuring assembly.
The Projected Moiré technique is similar to the Shadow Moiré technique since the grid, positioned between the camera and the object, has a function similar to the shadow of the grid in the Shadow Moiré technique. However, a further drawback of the Projected Moiré technique is that it involves many adjustments, and therefore generally produces inaccurate results since it requires the positioning and tracking of two grids. Furthermore, the second grid tends to obscure the camera, preventing it from being used simultaneously to take other measurements.
The use of methods based on “phase-shifting” interferometry allows measurement of the relief of an object by analyzing the phase variations of a plurality of images of the object after projections of a pattern thereto. Each image corresponds to a variation of the position of the grid, or of any other means producing the pattern, relative to the object.
Indeed, the intensity I(x,y) for every pixel (x,y) on an interferometric image may be described by the following equation:I(x,y)=A(x,y)+B(x,y)•cos(ΔΦ(x,y))  (1)where ΔΦ is the phase variation (or phase modulation), and A and B are a coefficient that can be computed for every pixel.
In the PCT application No. WO 01/06210, entitled “Method And System For Measuring The Relief Of An Object”, Coulombe et al. describe a method and a system for measuring the height of an object using at least three interferometric images. Indeed, since Equation 1 comprises three unknowns, that is A, B and ΔΦ, three intensity values I1, I2 and I3 for each pixel, therefore three images are required to compute the phase variation ΔΦ.
Knowing the phase variation ΔΦ, the object height distribution (the relief at every point h(x)y) relative to a reference surface can be computed using the following equation (see FIG. 1):
                              h          ⁡                      (                          x              ,              y                        )                          =                                            ΔΦ              ⁡                              (                                  x                  ,                  y                                )                                      ·            p                                2            ⁢                          π              ·                              tan                ⁡                                  (                  θ                  )                                                                                        (        2        )            where p is the grid pitch and θ is the projection angle, as described hereinabove.
The three images used by Coulombe et al. correspond to small translation of a grid relative to the surface of the object. The displacements of the grid are so chosen as to yield phase variations in the images. Coulombe et al. suggest obtention of the images by using a system that allows moving the grid relative to the object to be measured. A minor drawback of such a system is that it requires moving the grid between each take of images, increasing the image acquisition time. This can be particularly detrimental, for example, when such a system is used to inspect moving objects on a production line. More generally, any moving parts in such systems increase the possibility of imprecision and also of breakage.
A method and a system for three-dimensional inspection of an object free of the above-mentioned drawbacks of the prior-art is thus desirable.